Connecting 2 iMacs over a wifi network

I am trying to connect two iMacs that are sitting next to each other via wifi so I can transfer files back and forth. They are on the same wifi but have different operating systems. One has Cataelina and one has High Sierra. They do not "see" each other at all. I have even tried to connect by "connect to server" and using the smb address. The computer that has High Sierra can see another iMac in the house but not the one sitting right next to it. The one with Catelina cannot see the third iMac. I am completely baffled (note I have a very very rudimentary knowledge of networking).

Help? Please. Transferring files back and forth via USB drive is a pain in the rear.